But I just rolled a new character, it's early game and I've popped over to Whiterun to get dragons existing in the game world. It's a light armour character and the Jarl gives me 'hide armour of the squire' +15 to light armour skill. I've already got leather armour which of course gives me a higher armour rating, hide being pretty low end. So I'm suddenly wondering if it's better to have the higher armour rating or the +15 bonus to light armour skill. And the thought occurs I'm not sure exactly what the difference is.
Any help?

Each skill level increases your armor rating by 0.4%. I guess it's additive, so 15 points will give you 6% boost. Assuming that you only wear hide armor and the rest stays leather, the difference probably won't be big enough to compensate for the 6 points you're losing from not wearing a full leather set.

Anyway, the bonus is automatically calculated, so you can just put on both armors and check which one gives you higher overall Armor Rating.

Is it me or are most of the gifts the jarls give you pretty worthless?

It seems to depend on which Jarl and when you get the thing, but yeah, a large percentage of the gifts are basically disenchant bait. In general, anything gotten really early or really late will be completely useless, while the stuff in the middle will depend on what you get. I had one character (since deleted) in the mid-30s get a Glass shield from Balgruuf, which was actually better than the Elven stuff he had at the time, but said character was an archer and couldn't use it. Had he been a melee specialist it would have been an awesome gift, as it had a good +Block enchantment on it and his Enchanting skill was pretty poor at the time.